'SELECT id, custrecord_pcsq_customer, ' +
      'custrecord_pcsq_type_check, custrecord_pcsq_fields from customrecord_parent_child_sync_queue ' +
      'WHERE custrecord_pcsq_status = "Pending" AND custrecord_pcsq_process_type = "Initial"; '

This error occurs: "name":"SSS_SEARCH_ERROR_OCCURRED","message":"Search error occurred: Unknown identifier '\"Pending\"'. What is wrong with this query? Status and Process Type field both are free form text.

Try single quotes around Pending and Initial

and the whole query in double quotes?

Sure, you can use double quotes around the whole query string, or escape the single quotes, or use backticks to create a template literal with variable interpolation instead of concatenation.

Use 2.1 and use template literals, will make it easier to read/write

It worked. Thanks @Clay Roper @Sandii